final variables, methods and classes

• final variable — can be assigned only once. A static final primitive or String is a constant (static final int MAX = 100;).
• final method — cannot be overridden by subclasses.
• final class — cannot be extended (String, Integer and records are final).

A final reference cannot be pointed at another object, but the object can still change:

final List<String> names = new ArrayList<>();
names.add("Asha");               // allowed: the list object changes
// names = new ArrayList<>();    // compile error: reassigning the reference

For a truly immutable object, make the class final, make all fields private final, provide no setters, and copy mutable inputs and outputs (or use List.copyOf). final fields also get a memory-model guarantee: once the constructor finishes, other threads see their correct values.